The use of balancing springs to counterbalance weights in various applications, for example in hydraulic and mechanical systems, has meant that components can be made smaller and less expensive and that they, in some cases, can be eliminated altogether.
A common field of application for balancing springs is in the counterbalancing of industrial robot arms, wherein the spring counteracts the gravitational forces of mutually movable robot parts. By virtue of the fact that the balancing spring compensates the dead weight of the robot arm, the latter can be optimized to enable lifting of large and heavy loads at maximum reach and the drive components for the robot can be of a smaller and more compact design.
Various types of balancing springs are used for counterbalancing industrial robots, for example springs of the hydraulic cylinder type, mechanical springs and gas springs.
The use of balancing springs, which are based on gas spring technology, offers a number of advantages as compared with mechanical springs and springs of the hydraulic cylinder type. A balancing gas spring has a higher efficiency, among other things, than a comparable hydraulic spring. A gas spring is capable of producing a greater force with minimal built-in dimensions than a mechanical spring. Furthermore, the balancing pressure of the gas spring can be regulated in a simple manner by adjusting the gas filling pressure in the gas spring.
Gas leakage will occur in all gas springs over time, even if they are sealed. Such gas leakage leads to a reduction of the gas pressure in the spring. In many industrial applications, such as press tools for steel molding, which is the most common field of application for the gas spring, gas leakage is not a problem since the gas springs are serviced more or less regularly and repaired when needed.
Gas springs are also sensitive to variations in the ambient temperature. When the temperature changes, so does the gas pressure inside the spring. The pressure in the spring increases with higher temperatures, and the pressure in the spring decreases with lower temperatures.
Because of the drawbacks stated above, gas springs have not been considered ideal for use as counterbalancing springs in industrial robots. There is an expressed need for a maintenance-free gas spring, which maintains a correct gas pressure in the spring at any given moment regardless of the ambient temperature, so that unwanted stoppage of the robot, and thus of production, occasioned by a need for servicing the spring can be avoided.
EP 1905551 discloses a balancing unit for an industrial robot, in which the balancing spring is a mechanical spring. The spring is supplied with pressurized air to prevent moisture and dirt from penetrating into the spring unit. However, the operation of this mechanical spring, just like the operation of a gas spring, is impeded by temperature variations and at high temperatures the operation has to be interrupted to allow for cooling, which results in unwanted production stops.
U.S. Pat. No. 6,408,225 discloses an arrangement for balancing the weight of a robot arm comprising a gas spring. The pressure in the spring is measured and used to control the movements of the robot and to automatically disconnect the robot drive in case the pressure exceeds/falls below a predetermined pressure limit as a result of temperature variations or gas leakage, so as to avoid damage to the robot. The robot then has to be serviced in order to restore the gas pressure in the spring before it can be put back into operation. With this system, the unit has to be stopped to restore the pressure in the spring, which affects production in a negative way.
There is thus a need for an improved balancing spring, whose function is unaffected by temperature variations in the operating environment and which is maintenance-free, so that the number of unwanted production stoppages can be reduced to a minimum.